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This paper presents an original design option dedicated to output-filtered pulsewidth modulation inverters used in uninterruptible power systems, embedded networks, or motor drives operating on long cables. It is based on the coupling of interleaved inverter ceils by means of intercell transformers (ICTs) and is characterized by very good dynamic behavior on load transients, which is an important issue in such applications. ICT solutions are compared with the classical interleaved solutions using separate inductors. A specific design method is developed to achieve this comparison based on toroidal core shapes. The results obtained with a 600 V, 7.5 kW inverter show the usefulness of ICT-based designs, especially when nanocrystalline magnetic material is used; high specific power and very good dynamic performances are obtained. Index Terms--Embedded converters, intercell transformer (ICT), interleaved inverters, uninterruptible power system (UPS).

This paper presents the developments and the results of an analytic method elaborated to design isolated intercell transformers. These particular magnetic components are intended for multicell interleaved flyback converters recently proposed and well-suited to low-voltage, high-power applications. A specific 2-D model to evaluate the winding losses is included in the design process. The results, obtained for standard core shapes, allow defining the ranges of the main parameters, number of cells, and switching frequency, required for future realizations. Index Terms--Flyback converter, intercell transformer (ICT), interleaved converters, interphase transformers.

An analytic formulation dedicated to the design of high-frequency power transformers is presented. This approach is based on the area product, and homothetic core shapes are considered. The temperature increase of the transformer is one of the main input data. Winding losses including the effect of eddy currents are calculated as well as core losses. An original model of magnetic losses valid over a wide frequency range of the common ferrite materials is introduced. The analytic formulation is validated by design examples concerning transformers that have been realized and tested. It is finally applied to study the evolution of power density versus operating frequency or power for different winding configurations or magnetic materials. Index Terms--AC resistance, winding losses.

An original converter composed of interleaved flyback is presented. The ceils are interconnected through intercell transformers replacing the standard multi-winding inductors used as insulation and storage magnetic device in the flyback. The combination of the interleaving effects and of the intercell transformer characteristics allows eliminating the mains limitations of the standard flyback due to the low performances of multiwindings inductors. In a first part, the different ways to interleave galvanic insulated converters as forward or flyback are briefly recalled. The second part presents the intercell transformers and shows that they can be used to interconnect interleaved converters cells. In the third and last part, this particular magnetic interconnection is applied to interleaved flyback cells and leads to create an 'intercell transformer flyback'. Its specific properties and advantages are described. Experimental results, obtained on a 2-kW test bench are finally presented. Index Terms--Coupled inductors, flyback, intercell transformer, interleaved converters, interphase transformers.

This paper details the digital implementation of a new observation strategy of the flying capacitor voltages dedicated to stacked multicell converters (SMC). This particular topology relies on the use of flying capacitors so as to equally share the voltage constraint on several semiconductors and increase the number of output levels. However, the SMC appropriate operation is guaranteed by the stability of the flying capacitor voltage levels, and the current flow creates variations of those quantities. The idea is therefore to develop an accurate estimation technique that avoids the utilization of differential sensors and allows the active control of the intermediate voltages. The observation has to meet several criteria. It must particularly reproduce in a quasi-analog way the dynamic of those quantities to enable the proper operation of the active control algorithm. The actual research scope thus focuses on the sliding mode observation of the flying capacitor voltages by means of phase current measurements. This paper summarizes the observer design, its digitizing, and the field-programmable gate array (FPGA) implementation. It also presents an interesting Very High Speed Integrated Circuit Hardware Description Language (VHDL)-Saber Cosimulation tool that allows validating the VHDL code before being downloaded in the FPGA of the physical system. The last section illustrates and emphasizes the performance and ruggedness of the implemented algorithm through promising experimental results. Index Terms--Cosimulation tool, digital signal processing, field-programmable gate arrays (FPGAs), power system state estimation, pulsewidth-modulated power converters, stacked multicell converter (SMC).

Interleaved power converters are now used in many different conversion systems involving various topologies (series or parallel) and related to different fields or loads. This paper deals with interleaved parallel commutation cells using coupling transformers with a possibly high number of cells. The first part of the paper is a reminder of the basics of magnetic couplers addressing monolithic as well as distributed implementations. The limits associated with the conventional supply of such couplers (supply voltages forming a direct polyphase system) are described. In the second part of the paper, an optimized voltage supply improving the performances of the system is introduced. Experimental results obtained on a seven-cells test bench validate the approach. Index Terms--Power converters, uninterruptible power supply (UPS) voltage regulator module (VRM).

As fuel cells are likely to be used in many future applications, dedicated power converters must be developed and optimized. A thorough knowledge of the fuel cell operation is thus required for power electronics engineers. This paper proposes a theoretical and experimental study of the behavior of a fuel cell stack subject to current harmonics. The fundamental role of the internal double layer capacitor is demonstrated. Index Terms--Current harmonics, fuel cell.

Modulation of flying capacitor and stacked multicell converters is complicated by the fact that these converters have redundant states that achieve the same phase leg voltage output. Hence, a modulator must use some secondary criteria such as cell voltage balancing to fully define the converter switched state. Alternatively, the modulator can be adapted to directly specify the cell states, such as has been proposed for the harmonically optimal phase disposition (PD) strategy. However the techniques reported to date can lead to uneven distribution of switching transitions between cells, and the synthesis of narrow switched phase leg pulses. This paper presents an improved strategy that decouples the tasks of voltage level selection and switching event distribution. Conventional PD and centered space vector pulsewidth modulation (CSVPWM) strategies are used to define the target voltage level for the converter, and a finite state machine is then used to distribute the transitions to the converter cells in a cyclical fashion. Experimental results for a four-level flying capacitor inverter are presented, verifying that the natural balancing properties of this converter has been preserved, the cell switching utilization is equal and the expected harmonic gains of PD and CSVPWM compared to phase shifted carrier PWM have been achieved. Index Terms--Flying capacitor, multicell, multilevel space vector modulation (SVM), natural balance, phase disposition (PD), stacked multicell (SMC).

Peak current control is a strategy widely used to control power electronic systems such as forward converters or two-level choppers. This strategy is characterized by constant switching frequency with good dynamic performance. This paper presents the application of this strategy to a multilevel voltage-source converter. The algorithm ensures that the converter output current follows the target reference, but it must be modified to include voltage-level selection within the multilevel converter. The principle of operation is introduced, and the implementation of the algorithm using a field-programmable gate array is demonstrated. Simulation and experimental results are presented to verify the control technique. Index Terms--Field-programmable gate array (FPGA), multicell converters, multilevel converter, peak current control (PCC).

This paper focuses on the development of the natural balancing theory for the p-cell case. It describes the relationship between the models for different numbers of cells in a generic model for a p-cell multicell converter. The model discussed is based on the same principles that were used to develop the two-cell model in [16], except that the mathematics is much more involved. The same conclusions that were found to be true for the two-cell case was also found to be true for the general case of p cells. These conclusions include that the natural balancing mechanism of multicell converters depends on the overlap of the groups of harmonics of the switching function as well as on the load impedance. It will also be shown that the self-balancing mechanism ensures safe operation under most operating conditions where a high enough switching frequency is chosen and the load is not purely reactive. Two new aspects of the balancing theory were identified in the p-cell case: 1) for fixed duty-cycle modulation there exists certain values of the duty-cycle that causes the natural balancing mechanism to fail and 2) for p-cell converters the balance booster concept can be extended to a number of balance boosters tuned to multiples of the switching frequency. A 'Design Tool' based on the balancing theory was developed to aid practicing engineers in designing multicell converters. Index Terms--p-cell, two-cell case.

The multicell converter topology is said to possess a natural voltage balancing property. This paper is the first of a two-part series in which multicell converters are modelled for the general case of p-cells. This paper focuses on the development of the natural balancing theory for the two-cell case. An understanding of the two-cell case is fundamental to understanding the general balancing theory. The switching functions used in switching these converters are mathematically analyzed. Equivalent circuits are derived and presented. The switching and balancing properties of these converters are mathematically analyzed. The main conclusion of the analysis is that the natural balancing of these converters are influenced by three factors namely, the harmonic content of the reference waveform, the switching frequency and the load impedance. Mathematical tools are presented that can help designers to predict if balancing problems would occur for a particular set of operating conditions. As a result of the detailed understanding of the balancing mechanism that is gained through this theory it is shown that by adding a balance booster, the load impedance can be manipulated to improve the natural balancing of the converter. Simulation results are included to verify the presented balance theory and properties. Index Terms--Insulated gate bipolar transistor (IGBT), p-cells.

It is known that the optimal carrier based approach for modulating a multilevel converter is to use a phase disposition (PD) carrier arrangement with a common mode offset added to the reference waveforms to centre the implicitly selected space vectors. However, this strategy does not fully utilize all available voltage levels at lower modulation depths, with an odd level system only using odd voltage levels and an even level system only using even voltage levels as the modulation depth varies. Recent work has suggested that this is not the harmonically optimal approach at reduced modulation depths. This paper shows how up to a 40% reduction in harmonic distortion can be achieved if all available voltage levels are used throughout the linear modulation range. The improvement is achieved by adding a simple (1/2) carrier magnitude common mode dc offset in key modulation regions, which allows the converter to use all available voltage levels. The paper uses analytical spectral decomposition and harmonic flux trajectory analysis to propose a theoretical basis for this improvement, and to determine the precise points at which the (1/2) carrier magnitude offset should be added to achieve the harmonic improvement. Index Terms--Double Fourier series, harmonic flux trajectory, multilevel converter, pulsewidth modulation (PWM), space vector modulation (SVM).

Power-factor-corrector (PFC) converters are now commonly used in low-power supply systems connected to ac networks. In addition to their basic PFC properties, they constitute the best technical solution to directly obtain the compatibility with a large range of ac voltages provided by different distribution networks around the world (typically, from 85 to 265 V rms). It is the main application of these converters. This function is achieved with an additional cost and volume, and this extra price to pay needs to be minimized. In this context, we first recall the main approaches of the design of conventional PFCs based on the boost converter topology. We emphasize the different critical points of the sizing that mainly concern the input choke and the silicon devices, in regard with the choice of the switching frequency. Few ways of improvement are then presented for these conventional PFCs. In a second part, always in the context of a large input voltage range, we consider the possibility to introduce the multi-level concept to reduce the input choke and filters. A solution using a multilevel Flying Capacitor Two-Cell converter is presented. It is shown that it can lead to a significant increase of the performances, with several options of sizing which can be suited to different requirements of the applications. Index Terms--Flying capacitor multicell converter, flying capacitors, multilevel converter, power-factor corrector (PFC).

The auxiliary resonant commutated pole (ARCP) multiceLl converter is a soft-switched variant of the multicell converter. It is shown in this paper that the ARCP technique can be very efficiently used in multicell flying capacitor converters. The main properties of the resulting soft switching multicell converter are very similar to those of the hard-switched version. They are presented and validated by simulations as well as experimental results. In practice, due to the damping, the ARCP multicell converter can suffer from switching faults as in two-level ARCP inverters, but in the case of a multicell converter failures can occur in different cells. So, the main control strategies are evaluated and the switching process is discussed step by step, taking account of the main imperfections of actual control circuits. When they can occur, the switching faults are described and analyzed. Finally, an original quasisoft switching control that should give a high safety of operation is proposed. Experimental results obtained with a 900 V-100 A ARCP multicell inverter leg are given and the performances are compared with those of a hard-switched multicell inverter leg of the same rating. Index Terms--ARCP, dual-thyristor, multicell flying capacitor inverter, safe control, soft-switching, switching faults.

When high dynamic performances are desired, multicell converters are an option, but they may require extra control loops. However, cost reduction does not allow using additional sensors, and low cost estimators and observers have to be developed. Field-programmable gates arrays (FPGAs) with high sampling frequency seem to be able to perform such functions. The aim of this paper is to show how co-simulation helps designing such estimators and predicting their performances. Index Terms--Co-simulation, digital control, estimator, observer, power electronics.

The aim of this paper is to explain the intrinsic short-circuit tolerance of an IGBT multicell inverter when a commutation failure occurs. Such a failure may either be a wrong gate voltage (malfunctioning of the driver board, auxiliary power supply failure, dv / dt disturbance) or an intrinsic IGBT failure (over-voltage/avalanche stress, temperature overshoot). IGBT stresses are studied and show that no opening of the bonding can appear and consequently no risk of explosion. That is why, owing to the imbricated cells structure, an IGBT short-circuit failure may be withstood for a few switching periods, with nevertheless nonoptimized output waveforms. The design, the lab-test of a sensor able to perform monitoring as well as the failure diagnosis are also presented. This real-time diagnosis allows either a safe stop or a remedial control strategy based on the reconfiguration of the PWM modulator. The reconfiguration strategy enables decrease internal stresses and optimize the output shape. A fail-safe operating may be gained for high power applications. Index Terms--Fail-safe operation, faults, multicell converters, multilevel converters, redundancy.

The imbricated-cell multilevel converter is well suited to high power applications. It allows the series connection of n switches with natural voltage sharing between these switches enabled through the connection of n - 1 flying capacitors. This paper deals with the application of soft-switching on this topology; to date, only the hard-switching mode has been studied. The use of soft switching enables an increase of the switching frequency (resulting in the size reduction of the flying capacitors) without a decrease of the converter efficiency. Of the soft switching methods considered, the Auxiliary Resonant Commutated Pole (ARCP) technique was chosen due to the relative ease in which it can be incorporated into the converter topology. Furthermore, this technique offers numerous advantages: loss reduction, no added stress to the switches and compatibility with PWM control. The main properties of the ARCP multicell converter are the same as the hard-switched topology: an increase of the apparent output switching frequency and natural self-balancing of the flying-capacitor voltages. This paper presents the results of both simulations performed and measurements taken from an experimental set-up in order to study the viable system functioning. The introduction of soft-switching strongly complicates the theoretical study of the balancing mechanisms, however. As a result, the authors depend on simulations to validate the natural balancing effect during soft switching. Lastly, a general method of loss measurement is presented. Results show that the converter losses are reduced by at least 30%. Index Terms--ARCP, multicell converter, opposition method, soft switching, ZVS.

Multicell converters were introduced ten years ago and, over this period, their properties have been thoroughly analyzed. Since then, this concept has lead to some other innovative topologies which are briefly presented in this paper. Different ways to introduce soft switching in multicell converters are investigated. The concept of distributing power over several switches, giving more degrees of freedom and using less passive components, is extended further with the stacked multiceli topology. Finally, direct ac-ac converters using the multicell approach are described. Index Terms--AC-AC power conversion, insulated gate bipolar transistors, multilevel systems, power electronics, power systems harmonics, resonant power conversion, variable-speed drives.

The multicell converters introduced more than ten years ago make it possible to distribute the voltage constraints among series-connected switches and to improve the output waveforms (increased number of levels and apparent frequency). The balance of the constraints requires an appropriate distribution of the flying voltages. This paper presents some solutions for the active control of the voltages across the flying capacitors in the presence of rapid variation of the input voltage. The latter part of this paper is dedicated to the observation of these voltages using an original modeling of the converter. Index Terms--Kalman filtering, multilevel systems, nonlinear systems, power electronics, power systems harmonics.

Multilevel converters with series connection of semiconductors allow power electronics to reach medium voltages (1-10 kV) with relatively standard components. The increase of the number of semiconductors provides extra degrees of freedom, which can be used to improve different characteristics. This paper is focused on variable-speed drives and it is shown that with the proposed multilevel direct torque control strategy (DiCoIF) the tradeoff between the performances of the drive (harmonic distortions, torque dynamics, voltage step gradients, etc.) and the switching frequency of semiconductors is improved. Then, a slightly modified strategy reducing common-mode voltage and bearing currents is presented. Index Terms--Bearings (mechanical), delay estimation, industrial power systems, insulated gate bipolar transistors, multilevel systems, power electronics, state estimation, torque control, variable-speed drives.

Component counts and oversimplified reliability rules may lead to the conclusion that multilevel converters are less safe than two-level converters, just because they use more components. A better approach might be to consider that they use a different arrangement of components and also that the consequence of faults may be very different. This paper is focused on the study of the consequences of faults in hard-switching and soft-switching multicell converters. Solutions to minimize the consequences of major faults are described. Index Terms--Fault currents, fault detection, fault location, fault tolerance, insulated gate bipolar transistors, multilevel systems, power electronics, power systems harmonics.

Multicell converters were introduced ten years ago. This paper summarizes the main results obtained through the research done on these converters over this period: topology and derivation of multicell choppers and voltage-source inverters, harmonics cancellation, open-loop modulation techniques, self-balancing understanding, and optimization. During this same period, industrial products using this technique have been developed and the corresponding know-how has contributed to the field in various ways, such as modular design, low inductance busbars, specific drivers, etc. Index Terms--Industrial power systems, insulated gate bipolar transistors, multilevel systems, power electronics, power systems harmonics, variable-speed drives.

A new strategy for direct torque control with imposed switching frequency (DiCoIF) is proposed. This strategy was specially designed to operate with a multicell (flying capacitors) inverter with any number of levels, which means it can also be used for standard two-level inverters. This approach combines the well known advantages of the multicell inverter with those of a direct torque controlled (DTC) based strategy. It is shown that the multicell topology presents enough degrees of freedom to control both torque and flux with very low ripple and high dynamics on one hand, and to impose the switching frequency and the capacitors voltage balance on the other hand. Experimental and simulation results, obtained with a standard two-level inverter and with a four-level multicell inverter, are presented and discussed. Finally, a comparative analysis either with the classical DTC and field oriented techniques is carried out. Index Terms--AC motor drives, frequency control, multilevel systems, power electronics, torque control.

This paper is related to faults that can appear in multilevel (ML) inverters, which have a high number of components. This is a subject of increasing importance in high-power inverters. First, methods to identify a fault are classified and briefly described for each topology. In addition, a number of strategies and hardware modifications that allow for operation in faulty conditions are also presented. As a result of the analyzed works, it can be concluded that ML inverters can significantly increase their availability and are able to operate even with some faulty components. [ABSTRACT FROM AUTHOR]